The present invention relates to a method for producing a proton-conducting, polyazole-containing membrane.
Polymer electrolyte membranes (PEM) are already known and, in particular, are used in fuel cells. Sulfonic acid-modified polymers, in particular perfluorinated polymers, are often used for this purpose. One prominent example of these is Nafion™ from DuPont de Nemours, Willmington USA. Proton conduction entails a relatively high water content in the membrane, typically amounting to 4-20 molecules of water per sulfonic acid group. Not only the necessary water content, but also the stability of the polymer in conjunction with acidic water and the reaction gases hydrogen and oxygen, conventionally limit the operating temperature of the PEM fuel cell stack to 80-100° C. Under pressure, operating temperatures can be raised to >120° C. Otherwise, higher operating temperatures cannot be achieved without a drop in fuel cell performance.
However, for systems engineering reasons operating temperatures of higher than 100° C. in the fuel cell are desirable. The activity of the noble metal-based catalysts present in the membrane-electrode unit (MEU) is substantially better at elevated operating temperatures. In particular when hydrocarbon “reformates” are used, the reformer gas contains considerable quantities of carbon monoxide which conventionally have to be removed by complex gas preparation or purification. The tolerance of the catalysts to CO contamination increases at elevated operating temperatures.
Furthermore, heat arises during fuel cell operation. However, cooling these systems to below 80° C. may be very expensive. Depending on power output, the cooling devices may be of substantially simpler design. That means that, in fuel cell systems which are operated at temperatures of above 100° C., the waste heat is distinctly more readily utilisable and efficiency of the fuel cell system can be increased by combined heat and power generation.
Membranes with new conductivity mechanisms are generally used to achieve these temperatures. One approach is to use membranes which exhibit electrical conductivity without the use of water. The first promising development in this direction is presented in publication WO 96/13872. This in particular proposes using acid-doped polybenzimidazole membranes which are produced by casting.
Documents DE 102 46 459 A1, DE 102 46 461 A1 and DE 102 13 540 A1 describe further developments of this type of membrane.
DE 102 46 461 A1 discloses proton-conducting polymer membranes which are obtainable by a method which comprises the steps:    A) producing a mixture comprising polyphosphoric acid, at least one polyazole (polymer A) and/or at least one or more compound(s) which, on exposure to heat according to step B), is/are suitable for forming polyazoles,    B) heating the mixture obtainable according to step A) under inert gas to temperatures of up to 400° C.,    C) applying a layer using the mixture according to step A) and/or B) onto a support,    D) treating the membrane formed in step C) until it is self-supporting,wherein at least one further polymer (polymer B), which is not a polyazole, is added to the composition obtainable according to step A) and/or step B), the weight ratio of polyazole to polymer B being in the range from 0.1 to 50.
DE 102 46 459 A1 relates to proton-conducting polymer membranes based on polyazoles containing phosphonic acid groups which are obtained by a method which comprises the steps:    A) mixing one or more aromatic and/or heteroaromatic tetra-amino compounds with one or more aromatic and/or heteroaromatic carboxylic acids or the derivatives thereof which contain at least two acid groups per carboxylic acid monomer, wherein at least a proportion of the tetra-amino compounds and/or of the carboxylic acids comprises at least one phosphonic acid group, or mixing one or more aromatic and/or heteroaromatic diaminocarboxylic acids, at least a proportion of which comprises phosphonic acid groups, in polyphosphoric acid, to form a solution and/or dispersion,    B) heating the solution and/or dispersion obtainable according to step A) under inert gas to temperatures of up to 350° C. while forming polyazole polymers,    C) applying a layer using the mixture according to step A) and/or B) onto a support,    D) treating the membrane formed in step C) until it is self-supporting.
DE 102 13 540 A1 relates to proton-conducting polymer membranes based on polyvinylphosphonic acid which are obtainable by a method which comprises the steps:    A) dissolving a polymer, in particular a polyazole, in phosphonic acid containing vinyl,    B) forming a planar structure using the solution according to step A) on a support    C) applying a starter solution onto the planar structure formed according to step B) and    D) polymerising the vinylphosphonic acid present in the planar structure according to step C).
In these methods, it is intended for the planar structure to be formed [step C) in DE 102 46 461 A1, step C) in DE 102 46 459 A1, step B) in DE 102 13 540 A1] by means of per se known measures, such as for example casting, spraying, knife coating, extrusion, which are known from the prior art for polymer film production. However, no further indications as to the exact procedure are to be inferred from the documents.
Producing the above membranes by casting, spraying or knife coating is unfortunately very complex and costly. It requires the use of large quantities of solvent to dissolve and apply the polymer onto the support, which solvent must subsequently be removed and recovered. The method is furthermore very time-consuming and permits only a low space-time yield. The fluctuations in quality which are frequently to be observed between different production batches constitute an additional problem. Furthermore, processing polyazoles with comparatively high molecular weights is particularly difficult due to the relatively poor solubility of these polymers, the increasing non-uniformity of the corresponding solutions and the increasing formation of bubbles.
Extruding the mixtures to form the corresponding planar structures is also non-trivial. The problem in particular arises that, due to the comparatively high temperatures, the polyazoles continue to condense, so forming polymers with ever higher molecular weights, whereby the properties of the polymers and the membranes, if they can even be obtained, are significantly impaired. Furthermore, due to the high molecular weight, processing of the polyazoles becomes increasingly difficult, such that in many cases membranes can no longer even be obtained. It is at present not possible to produce membranes with high levels of quality and reproducibility.